1. Field of the Invention
The present invention relates to an environmental and thermal barrier overlay coating system is described for advanced copper alloys for possible use in reusable launch vehicles and other similar applications. In particular, the present invention addresses issues related to the design and deposition of suitable coatings and bond coating technology for protecting an advanced copper alloy known as GRCop-84.
2. Description of Related Art
Copper alloys, containing chromium and niobium, are being considered for use as combustor and nozzle liners in reusable launch vehicle (RLV) applications. The combustion chamber of a rocket engine is exposed to a high heat flux and gas pressure. Experience has shown that unprotected copper alloys undergo a degradation in properties and life during use in rocket engines due to a process called blanching, where the material undergoes repetitive cycles of oxidation and reduction by the hydrogen/oxygen combustion mixture finally leading to failure or reduced useful life of the component.
Other complications arise due to thermal fatigue of the metallic cooling channels carrying liquid hydrogen (LH2). This process leads to a process known as a “dog house” effect, where initially square coolant channels attain the shape of a dog house and eventually fail with the subsequent loss of LH2 and poor engine performance. This problem is especially critical since the wall thickness of the cooling channels is about 1.0 mm.
The two characteristics of the problem are environmental attack of the surfaces of the copper alloy combustor liners and nozzles and thermal fatigue of these components.
Wear and abrasion resistant coatings have been developed for copper alloys, such as those disclosed in Sugawara et al., U.S. Pat. No. 6,040,067. Chiang et al. proposed using Cu-30+10 vol. % Cr as blanch resistant coating for copper alloy rocket engine combustion chamber linings (U.S. Pat. No. 5,557,927). An oxidation resistant coating having a copper-aluminum alloy and a cobalt-based alloy diffusion barrier has been reported (See, U.S. Pat. No. 6,277,499, by Beers et al.) and a method of fabricating the combustion chamber for a rocket engine with suitable metallic or ceramic coating by low pressure or vacuum plasma spraying, where the composition of the component was functionally graded from the protective coating on the inside to a Cu-8(at. %)Cr-4(at. %)Nb (GRCop-84) alloy on the outside. (See, U.S. Pat. No. 6,314,720, by Holmes et al.).
Additionally, nickel aluminide coatings, NiAl, having intermetallic compounds, have been used to coat nickel-based superalloy turbine blades and vanes in aircraft engines as a bond coat (See, Bruce et al., U.S. Pat. Nos. 5,981,088 and 6,352,788) or as a top coat (See, Rigney et al., U.S. Pat. Nos. 6,153,313 and 6,291,084).
However, all of the above cited coatings have limitations. Several have low melting points and therefore are unlikely to be useful as overlay coatings for rocket combustion chamber liners. Some are specifically applicable to nickel-based superalloy turbine airfoils. The approach adopted by Holmes et al. does not involve overlay coatings, since it involves fabricating the entire component by gradually changing the composition from the inner to the outer surface. However, the ability to fabricate large RLV combustor components by such a technique is unproven. The Cu-30(vol. %)Cr coating proposed by Chiang et al. is unlikely to be effective at temperatures above 873 K in applications requiring a large number of mission cycles due to the increased possibility of spallation and coating degradation.
None of the cited prior art deals with the development of NiAl overlay coatings and methods of applying them on advanced copper alloys, such as Cu-8(at. %)Cr-4(at. %)Nb. This new overlay coating should provide a suitable high temperature environmentally resistant thermal barrier coating for reusable launch vehicle rocket engines.